Problem: 7 folders cost $7.21. Which equation would help determine the cost of 10 folders?
Solution: There are several equations that could help determine the cost, each with a slightly different approach. We can write the fact that 7 folders cost $7.21 as a proportion: $\dfrac{7}{\$7.21}$ Let $x$ represent the unknown cost of 10 folders. Since 10 folders cost $x$ , we have the following proportion: $\dfrac{10}{x}$ The cost changes along with the number of folders purchased, and so the two proportions are equivalent. $\dfrac{7}{\$7.21} = \dfrac{10}{x}$